Symmetrical Quasi-Reflectionless SAW-Based Bandpass Filters With Tunable Bandwidth

Surface acoustic wave (SAW)-based bandpass filters (BPFs) with symmetrical quasi-reflectionless characteristics and continuously tunable bandwidth (BW) are reported. They are based on in-series-cascaded symmetrical quasi-reflectionless acoustic-wave (AW)-lumped-element resonator (AWLR)-based networks whose input and output ports are connected to resistively terminated AWLR-based bandstop filter (BSF) sections. The proposed concept is presented through the coupling routing diagram (CRD) formalism that allows their design using coupled-resonator-based synthesis. It is shown that by incorporating variable reactance elements in their BPF sections, continuous-type BW tuning can be realized. In addition, the fractional BW (FBW) of the filter can be designed to be wider than the electromechanical coupling coefficient k(t)(2) of its SAW resonators. This allows enhanced-FBW passbands to be obtained. For proof-of-concept validation, two prototypes were designed and measured. They include a single-stage prototype with 1.9:1 continuously tunable BW and a static two-stage with effective quality factor > 6500, FBW of 1.2k(t)(2), and return loss > 15 dB throughout its passband and stopband ranges.